Magnetic pressure indicator

ABSTRACT

A magnetic differential pressure indicator is provided for detecting and indicating a pressure differential thereacross that is greater than a predetermined value in a fluid system. A loose nonmagnetic ball detent prevents resetting of the magnetic indicating means after actuation.

[ .Ian. 15, 1974 3,077,176 2/1963 Pallet 3,077,854 2/1963 Pall..........3,140,690 7/1964 Siebel 3,187,71l 6/1965 Compolong..... 3,212,471l0/l965 Willis..................

Primary Examiner-Louis Capozi Attorney-James & Chapman [57] ABSTRACT Amagnetic differential pressure indicator is provided for detecting andindicating a pressure differential thereacross that is greater than apredetermined value in a fluid system. A loose nonmagnetic ball detentprevents resetting of the magnetic indicating means after actuation. v

20 Claims, 4 Drawing Figures United States Patent 1 1 SilverwaterBernard F. Silverwater, Plainview, NY.

Assignee: Pall Corporation, Glen Cove, NY.

[22] Filed: Jan. 31, 1972 [21] Appl. No.: 222,266

[52] US. 116/70, 210/90 Int. Cl. G011 19/12 Field of Search 116/70, 117;210/90; 200/81; 137/554; 340/366 References Cited UNITED STATES PATENTS6/1960 ll6/70 X MAGNETIC PRESSURE INDICATOR [75] Inventor:

we N:

PATENTH] JAN I 5 I974 SNEEI 1 BF 2 MAGNETIC PRESSURE INDICATOR In anysystem wherein a fluid such as hydraulic fluid or the like is passedthrough a filter, it is customary to provide means for indicating whenthe filter element has become loaded, and requires replacement. Sincethe pressure drop across a filter increases in proportion to theaccumulation of contaminants thereon, a suitable indication can beobtained by an indicator actuated when the differential pressure acrossthe filter reaches a predetermined value. Many types of such devices areavailable, in which the indicating means is actuated mechanically, orelectrically, or by other means.

One simple and very successful type of indicating device is the magneticpressure indicator of US. Pat. No. 2,942,572 to David B. Pall. In thisdevice, a first magnetic means is arranged to attract a second magneticmeans so long as the two means are separated by less than apredetermined distance. Bias means propels the second magnetic elementto an indicating position whenever that distance is exceeded. The firstmagnetic means is movable with a piston responsive to changes inpressure, and is normally biased towards the second magnetic means by apredetermined force. The second magnetic means is also movable with apiston, and while retained toward the first means by magnetic attractionwhen close enough thereto, is normally biased in a direction away fromthe first means by a force capable of overcoming the force of magneticattraction whenever the first and second means are separated by thepredetermined distance. The magnitude of the force of magneticattraction relative to the spring biasing force determines thepredetermined pressure differential at which the device is actuated.Thus, the device can be arranged to be actuated at any requireddifferential pressure, by simple adjustment of these forces.

In the usual form of this device, the second magnetic means is arrangedto project from the housing after ac tuation. In another form, thesecond magnetic meansis arranged to give an electric signal uponactuation. In

either case, the device can be reset merely by pushing the secondmagnetic means back toward the first magnetic means, into a positionwhere the magnetic force once again overcomes the spring-biasing force.If the differential pressure across the filter element has meanwhilebeen reduced, due for instance to a reduction: in the flow, the resetindicating means will thereupon remain in its normal position, and aperson unaware that the device has already given its signal would haveno way of knowing that the filter is no longer operative, and that thesystem isfunctioning on bypass. If the system is in operation on anaeroplane during flight, the resulting danger to the aircraft is verygreat, and may even result in loss of the aircraft.

In accordance with the invention, a magnetic differential pressureindicator is provided with means to prevent reset of the indicatingmeans after actuation without actually inverting the device, thusforestalling cancellation of the signal. The device features agravityactuated loose nonmagnetic ball detent which assumes anobstructing position in any normal orientation of the indicatorencountered under service, and prevents return of the indicating meansto its normal nonindicating position.

The term magnetic as used herein encompasses both-materials that arepermanent magnets and materials that are attracted by magnets, whetherpermanently or temporarily magnetizable thereby.

The device in accordance with the invention comprises a first magneticmeans spaced from and arranged to attract or repel a second magneticmeans or keeper so long as the two means or the first means and keeperare within their mutual magnetic fields of force, the second magneticmeans normally being retained in a first position, first bias means toretain the first magnetic means towards or away from the second magneticmeans or keeper, and second bias means to propel the second magneticmeans from the first position to an indicating position whenever thespacing between the two magnetic means is changed. The first magneticmeans is movable with or is a first piston in response to changes inpressure, and is normally biased toward or away from the second magneticmeans or keeper by a predetermined force. The second magnetic means ismovable with or is a second piston, and is retained toward the firstmagnetic means or keeper by magnetic attraction, when close enoughthereto, but is normally biased in a direction away from the first meansor keeper by a force capable of overcoming the force of magneticattraction to the first means or keeper whenever the first and secondmagnetic means or keeper are spaced by a predetermined distance.

As the loose nonmagnetic ball detent, there is provided a free-rollingball normally nesting in a recess opening into the space within whichthe second magnetic means or piston moves, and retained in the recess bythe second magnetic means or piston while the means or piston ismagnetically attracted to the first magnetic means or keeper. Wheneverthe second piston and second magnetic means move a distance away fromthe first magnetic means or keeper sufficient to expose the recess, theloose ball rolls into the space and freely assumes a position in whichit allows return of the second magnetic means only to a position wherethe spring bia'sing force is still capable of overcoming the magneticattraction between the second magnetic means and the first magneticmeans or keeper. The recess is so arranged with respect to the secondmagnetic means or piston that the ball cannot be returned to the recessby the second piston, in any normal orientation of the indicating deviceencountered under service. The ball has a diameter greater than thedistance within which the second magnetic-means is attracted by magneticforce to the first magnetic means or keeper.

The first and second magnetic means are each movable, and can be magnetsor attracted to magnets. The keeper is stationary, and is positionedtherebetween, spacing. them by a predetermined distance at least equalto the spacing dimension of the keeper, and can be a magnet or attractedby magnets, so that one at least of the magnetic means is attracted tothe keeper. The other can be attracted to the keeper or other magneticmeans, or repelled by the keeper or the other magnetic means.

At least one of the two magnetic means and keeper is a magnet.Preferably, two of these elements (in any combination) are magnets, andif desired, all three can be magnets, but three are not always asadvantageous as two. In addition, the keeper can be of nonmagneticmaterial.

If the keeper is a magnet and the first and second magnetic means aremerely attracted thereto, the device functions because the first magnetmeans when it moves from or moves towards the keeper changes themagnetic field between the keeper and the second magnetic means.

The keeper normally serves as the separating wall between the first andsecond magnetic means, and prevents fluid communication between thespaces within which the first and second magnetic means move. The keepercan be a piece fitted between these spaces, or an integral part of thehousing defining such spaces. If the spaces are bores, usually coaxial,the keeper can close off and separate the two parts of the bore fromeach other, and define a pair of blind bores within which the twomagnetic means move.

The first magnetic means can take any of several forms. It can, forexample, be a piston, or part of a piston, as illustrated in thedrawings, which can be in the form of a cylinder. lt can also take theform ofa piston or part of a piston of high surface area, such as aflexible disc or diaphragm, as shown and described, for instance, inU.S. Pat. No. 3,077,176, dated Feb. 12, 1963, to David B. Pall et al.,or a bellows.

The second magnetic means also can be in the form of a piston, or partof a piston. It can be arranged to project from the housing for theindicator, for a visual indication. It also can be arranged to actuate aswitch, as in U.S. Pat. No. 3,077,854 to David B. Pall, dated Feb. 19,1963, or to move an indicator, such as a pointer, or magnetic fibers,either directly or magnetically.

The biasing means for the first and second pistons can take the form ofa spring, such as a coil spring, a finger spring, a wave-form spring, aconical spring, or an annular disc spring, such as a Belleville spring.The biasing means can also be a third magnet, as disclosed in U.S. Pat.No. 3,140,690, to M.P.L. Siebel, in which case a spring can optionallybe included or omitted.

The drawings illustrate preferred embodiments of the invention.

FIG. 1 is a view in longitudinal section taken through a typicalpressure indicator according to the invention, in which the indicatingdevice is in the form of a piston having a cap arranged to project fromthe housing within which the device is disposed, and showing thepressure indicator in a nonactuated position.

FIG. 2 is another view in longitudinal section taken through thepressure indicator of FIG. 1, and showing the device in the actuatedposition.

FIG. 3 is a cross-sectional view taken on the line 3--3 of FIG. 1 andlooking in the direction of the arrows.

FIG. 4 is a view in longitudinal section through another embodiment ofthe pressure indicator of FIG. 1,

in which the piston indicating means is arranged to actuate a switch.

The pressure indicator of FIGS. 1 to 3 comprises first and secondmagnetic elements 1 and 2, respectively, coaxially mounted on oppositesides of a separating wall or keeper 3 of magnetic material within ahousing 4, which may be either of magnetic or nonmagnetic material.Elements 1 and 2 are positioned adjacent the wall or keeper 3, withmagnetic poles opposed, so that each is drawn toward the wall by theresulting force of magnetic attraction therebetween. Preferably, themagnetic elements 1 and 2 are composed of permanently magnetized metal,such Alnico Vl, Alnico Vlll, or ceramic magnetic material, or the like.If desired, however, element 2 may be formed of a suitable magneticmaterial such as iron, for example. Wall 3 is formed of a suitablemagnetic material such as iron, for example, but it can also be ofnonmagnetic material.

Mounted in a tubular piston 5, magnetic element 1 is slidably supportedin a cylindrical bore 6 in the housing 4, and is urged toward the wallor keeper 3 via bias means 7 which, in this embodiment, is a coilcompression spring. The tubular piston 5 and magnetic element 1 can alsobe all in one piece of magnetic material. In order to prevent fluid frompassing from the annular chamber 8 at one end of the bore 6 to the space9 at the other end, a liquid-tight seal is provided between the bore 6and the piston 5 by a O-ring l0 and a ring 11, of Teflon or othersuitable gasketing material. The seal may also be effected by closetolerances between the piston and bore, and the sealing rings omitted.The coil spring 7 is selected according to the desired actuatingpressure to permit the piston 5 to move away from wall 3 in the bore 6whenever the pressure at the end of the piston 5 exceeds the pressure atthe other end by an amount equal to the actuating differential pressure.

In this embodiment of the invention, the pressure indicator is arrangedto measure the pressure drop across a pressure-influencing component,such as a filter (not shown, but illustrated in U.S. Pat. No.2,942,572). Thus, fluid under pressure is applied to the component suchas a filter via an inlet line (not shown) and emerges on the other sideof the component through an outlet line (not shown). The bore 12 in thehousing 4 is in fluid-flow connection with the inlet line, andcommunicates fluid pressure in the inlet line to the annular chamber 8at one side of the piston 5, while the space 9 at the other end of thepiston 5 is in fluid pressure communication with the outlet line viaport 39. The difference in pressure between the inlet and outlet linesis a measure of the pressure drop across and therefore the clogging ofthe filter or other pressure-influencing com ponent, and the differentpressures are thus communicated to opposite sides of the piston 5.

If desired, in order to prevent dirt carried by the incoming fluid fromblocking the bore 12, and/or chamber 8, possibly obstructing movement ofthe piston, a suitable annular filter element 37 is inserted over theoutside of the housing 4 across the bore 12.

For ease in assembling of the piston 5, the bore 6 has its open endclosed off by the cap 13, and the base of the spring 7 is retained bythe cap against the abutment 14 of piston 5. The cap is permanentlyretained to the housing 4 by the flange 15. The spring can also beretained by the housing in which the indicator is installed, in whichevent the cap can be omitted.

The wall or keeper 3 is press-fitted in a leak-tight fit at the upperend of second bore 16 of the housing 4, or it may be an integral part ofthe housing 4; the bore 16 is coaxial with the bore 6, but it need notbe. Secured to the skirt end 17 of the magnetic element 2 by an annularskirt 18 is a cap 19, which extends over the exterior of the cylindricalprojection 20 of the housing 4, defining the bore 16 therewithin. Theskirt 18 is swaged onto the cap. Within an annular recess 21 in thehousing 4, surrounding projection 20, is a second bias means 22 which,in this embodiment, is a compression coil spring which extends from theinner face of the cap 19 within the flange 23 to the housing at the baseof the recess 21, and urges the cap 19 and the magnetic element 2 towhich it is attached away from the wall 3. This spring is selected sothat it is retained in the stressed condition while the magnetic element2 is held against the wall or keeper 3 by the attractive forces betweenthe magnetic elements 1 and 2, or between element 2 and keeper 3; thisattractive force between these elements is sufficient, so long as theadjacent poles of the magnetic elements 1 and 2 are separated by lessthan a predetermined distance, for example, one-sixteenth of an inch.Whenever the magnetic element 1 is moved away from the element 2 so thattheir adjacent poles are. separated by more than onesixteenth of aninch, however, the decreased force of magnetic attraction resultingtherefrom is overcome by the force of the spring 22, and the cap 19 andelement 2 are driven away from the wall 3.

In the wall 3 between the two elements 1 and 2 is a recess 24, withinwhich a nonmagnetic stainless steel ball 25 rests loosely. The diameterof the ball is greater than the predetermined distance over which themagnetic element 2 is attracted to the wall or keeper 3 with a magneticforce greater than that of spring 22, in this case, for example, 0.050inch. The face 34 of the wall 3 slopes away from the recess 24 towardthe sides of the bore 16. The face of the magnetic element or piston 2also slopes in a matching taper from the axial center of the magneticelement, so that the piston sits snugly against the face of the wall 3whenever the element 2 is magnetically attracted toward the element 1 orwall 3, and thus holds the ball 25 in the recess 24. Whenever the pistonand element 2 move away from the wall or keeper 3 a distance at leastequal to the diameter of the ball, the ball drops out of the recess andthen rolls down the sloping face of the piston 2, to come to restagainst the side of the bore 16, and remains in this position, by forceof gravity.

In order to prevent the cap 19 and magnetic element 2 from being drivencompletely out of the bore 16 upon actuation, the open end of theannular recess 21 is closed off by an annular disc 26, retained there bya spiral lock washer 27, snugly held in the narrow groove 28 at the endof the recess 21. The inner periphery 29 of the disc annulus closelyabuts, but does not touch, the cap 19. The flange 23 (which retains thespring 22) engages the disc 26 whenever the cap is thrust outwardly bythe spring, thereby preventing the cap from proceeding further than theposition shown in FIG. 2.

A conventional bimetallic element 30 is attached to spring retainingring 38, which is biased against the inside wall of the recess 21, justbelow the flange 23, retained there against the shoulder 31 by its ownspring force. The bimetallic element 30 is preferably comprised of twoarcuate inner and outer strip portions 32, 33 joined together, forexample by a weld, and both arranged to bend inwardly with decreasingtemperatures. At normal temperatures, element 30 has a minimum radiusgreater than that of the flange 23, and permits the cap 19 to ridethrough the aperture therebetween on actuation. However, at temperaturesbelow a preselected value, at which for example the fluid to be filteredincreases appreciably in viscosity, for example, 33 to 62 F., theelement 30 contracts inwardly, so that the inner strip 33 extends overthe face of the cap 19 just within the flange 23, thus engaging theflange and preventing actuation of the pressure indicator.

In operation, fluid pressure in the inlet line is communicated via theduct 12 to the space 8 of the cylindrical bore 6, urging the magneticelement 1 and piston 5 away from wall 3 against the force of the spring7 and the pressure from the outlet line, communicated to space 9 viaport 39. Whenever the difference between the inlet and outlet pressuresis greater than the force of the spring 7, the piston 5 is driven awayfrom wall 3 in the bore 6. After the magnetic element 1 is moved to aposition more than one-sixteenth of an inch away from the magneticelement 2, the attractive force therebetween is less than the force ofthe spring 22, and the magnetic element 2 is driven away from the wall 3until the cap 19 (which if desired, may be of a suitable eye-catchingcolor, such as red, orange or yellow) emerges through the aperture 34,and the flange 23 abuts the inside surface of the disc 26, where it isheld firmly by the spring 22, in the position shown in FIG. 2. At thesame time, the ball 25 is released from its recess 24, rolls along thesurface of the piston 2, and comes to rest against the side of the bore16.

The cap 19 in this position indicates that the pressure difference isgreater than the predetermined value in accordance with which the spring7 has been selected. As an example, the spring 7 may be arranged topermit the magnetic element 1 to be driven away from the wall 3 wheneverthe pressure difference exceeds 35 psi, and thus give a signal.

It will be apparent from FIG. 2 that the ball 25 prevents resetting ofthe magnetic element 2 to its original position, shown in FIG. 1. If themagnetic element 2 be pushed upwardly, the ball 25 is retained by thetapered face of piston 2 in position against the side of the bore 16.While the ball 25 moves with piston 2 along the bore until it engagesthe tapered face of wall 3, it thereafter prevents the piston from beingpushed any further than a distance from the wall 3 equal to the diameterof the ball 25, in this case, for example 0.050 inch. Since thisdistance, however, is greater than onesixteenth inch, more than thedistance over which the magnetic element 2 is attracted to the wall orkeeper 3 by a force greater than the biasing force of spring 22, thepiston 2 can be held in this position only while it is retained theremanually. As soon as the manual force is removed, the piston returns tothe indicating position shown in FIG. 2 under the force of the spring22.

At temperatures below 32 F., for example, the thermostatic element 30contracts, to move the inner face 33 of the strip 30 over the cap 19 toengage the flange 23. Thus, when the piston and the magnetic element 2seek to move away from the wall 3 under a differential pressureexceeding the biasing force of spring 7, resulting from increasedviscosity of the fluid, the inner edge of the strip 30 intercepts theflange 23, preventing a false indication of, for example, fllterclogging.

If desired, this pressure indicator may be utilized to indicate a totalpressure above atmospheric instead of a, pressure differential, bycommunicating port 39 opening to space 9 of bore 6 to the atmosphere.Similarly, an absolute pressure may be indicated by connecting the port39 and space 9 of bore 6 to a vacuum. A flexible boot 35, the enlargedperiphery of which is resiliently mounted in the annular recess 36 aboutremove the subcomponent from the installation on which the device ismounted, and invert it. In the inverted position, the ball 25 rollsfreely along the now inwardly-inclining face 34 of the wall 3, and dropsreadily into the recess 24. In this position of the ball, the piston 2and cap 19 can be returned to the position shown in FIG. 1. The devicecan then be reinverted, and is ready for reinstallation and reuse.

Normally, however, because of the fixed nature of the fluid flowconnections to the inlet and outlet lines, the pressure indicator cannotbe removed without closing down the system, such as would be done in acomplete reservicing, at the time a filter element is replaced. As apractical matter, therefore, resetting cannot be accomplished while thefluid system is still in use, which means that the indicator willcontinue to give an indication of the need for changing the filterelement or other component until in fact the filter element or componenthas been changed.

The pressure indicator shown in FIG. 4 corresponds to an invertedindicator of FIG. 1, and arranged to operate in the inverted position.In this case, the piston and magnetic element 2 are adapted to actuatean electrical signal, which, by choice of the appropriate electricalcircuits, in any conventional manner, can be adapated for example toshut off flow or to give a warning signal. 7

The indicator comprises a housing 40 with two magnetic elements 41 and42, coaxially mounted on opposite sides of the wall or keeper 43. Themagnetic elementor piston 42 has therewithin a recess 44 in which isloosely held a nonmagnetic ball 45, 0.060 inch in diameter, or any otherdiameter coupled to the strength of the magnetic field between the twoelements 41 and 42. The inner face of the piston 42 slopes from therecess 44 and toward the sides of the bore 46. The wall 43 is alsotapered from the center toward the sides of the bore 46. Thus, uponmovement of the piston 42 away from wall or keeper 43 over a'distancegreater than the diameter of the ball, the ball emerges from the recess44, and rolls downwardly along the sloping face ofthe wall or keeper 43,to come to rest against the side of the bore 46, in the position shownin phantom lines in FIG. 4.

To provide an electrical signal upon actuation of the indicator, aswitch 50 is mounted in a housing 51 which as shown is a part of theindicator housing 40, but need not be. Mounted on the switch is a lever53 pivotally supported at 54 with one end 55 projecting into the annularrecess 56 of the housing, defined about cylindrical projection 57, to apoint abutting but not touching the cap 58, above the flange 59.Positioned above the lever arm 53, a switch actuator 60 in the form ofapush button is movable by the lever arm 53 to actuate the switch 50whenever the cap moves in the annular recess 56 so that flange 59 comesinto contact with the end 55 of the lever arm 53, moving the arm 53against push button 60. To provide a remote indication of the actuationof the indicator, suitable conductors 61 from the switch 50 are arrangedin any conventional manner to complete appropriate electrical circuitswhenever the switch is actuated.

In operation, a source or inlet of high pressure is connected through aduct (not shown) to the space 62 at the top of the cylindrical bore 63,urging the magnetic element 41 and piston 47 away from wall 43 againstthe force of a spring (not shown), as in the device of FIGS.

1 to 3. The space beyond the piston 47 can be connected to the outletline (in which case the device acts in response to differentialpressure) or to the atmosphere, or to a vacuum. Whenever the differencebetween the pressures on opposite sides of the piston 47 is greater thantheforce of the spring, the piston 47 is driven away from wall 43 in thebore 63. Whenever the magnetic element 41 has been driven more thanonesixteenth of an inch from the wall 43, the indicator and switch willactuate. The magnetic element 42 and cap 58 will be driven by spring 64away from the wall 43 until the flange 59 abuts the end 55 of lever arm53, thus moving the arm against the push button 60, and actuating theswitch. At the same time, the movement of the piston 42 releases theball 45 from its recess 44, and the ball rolls down along the taperedwall 43, to come to rest against the side of the bore 46.

At the same time as the switch is actuated, the cap 58 emerges from thehousing, and also gives a visual signal of the actuation of theindicator.

If one attempts to reset the indicator by pushing the cap 58 downwardlyagainst the spring 64, the cap and piston 42 can only be pushed to thepoint at which they contact the ball 45, after which they are heldspaced from the wall or keeper 43 by a distance equal to the diameter ofthe ball, for example, 0.060 inch. This distance is greater than thedistance within which the magnetic element 41 exerts a magnetic force onwall or keeper 43 sufficient to hold the element 42 against the wall 43,and thus retain the cap 58 within the housing 51 against the force ofspring 64. 4 According to the distance the flange 59 is spaced from theend of cap 58, the lever arm may or may not be fully withdrawn fromcontact with the push button 60, when the piston 42 is pushed downagainst ball 45. In the device shown, the flange is so placed on cap 58that by pushing the piston and cap inwardly against the ball, the arm 53is released from contact with the push button 60, and the switch isdeactuated, but it is reactuated as soon as the cap and piston areallowed to return to the indicating position.

By appropriate location of an acutating flange like flange 59 on the cap58, nearer its tip, the flange does not become disengaged from the tipwhen the cap is pushed downwardly against the ball 45, and the arm 53does not pivot into a position away from actuation of the push button60, so that in this case even manual movement cannot result indeactuation of the switch The ball is replaced in recess 44 as in thedevice of FIGS. 1 to 3. The device is inverted, the ball rolls into therecess, and piston 42 is then pushed into its normal position againstwall 43. I

While in the devices shown the ball is retained in a recess either inthe wall between the magnetic elements or in the indicating piston, itwill be appreciated that the recess could also be positioned in the wallof the housing or in the side wall of the bore in which the indicatingpiston moves. In this event, the recess should be arranged to slopedownwardly, according to the normal orientation of the indicatingdevice, and positioned so that when the indicating piston is withdrawn asufficient distance, greater than the distance within which the magneticelements or keeper wall exert a sufficient magnetic attraction upon eachother to overcome the force of the spring, causing movement of thepiston into an actuating position, the ball will roll out from therecess into the bore, and remain there, in a position to preventresetting of the indicator. It is preferred, however, in most cases tohave the recess in an uppermost position in the wall or piston, in orderto prevent accidental return of the ball into the recess by a sidewisemovement or roll of the device, as could happen in the case of anaircraft.

in the embodiments shown in the drawings, the magnetic elements 1 and 2or 41 and 42 are arranged so as to attract each other, with opposedpoles. An equivalent resultcan be obtained by arranging the elements 1and 2 or 41 and 42 so as to be mutually repelling with like poles facingeach other. This requires only a small modification of the structureshown, that would be obvious to anyone skilled in this art with aknowledge of magnetic principles, using the principles of this inventionas discussed above.

In the embodiment shown in FIGS. 1 to 4, the magnetic element 1 or 41can be reversed, so that the south pole faces the south pole of magneticelement 2, or 42, and the element 1 or 41 relocated so that it is at theopposite end of the bore, in its normal position, with the spring 7relocated to the opposite side of the piston, to bias it in the oppositedirection. The pressure-sensing line connections 12 and 39 also have tobe reversed, so that outlet line pressure is sensed via bore 12 andinlet line pressure sensed via port 39. Now, a pressure differentialsensed by the piston 1 or 41 via bore 12 and port 39 tends to move thepiston towards wall 3 or 43, repels magnetic element 2 or 42 away fromwall 3 or 43, and eventually to an actuating position. In this type ofembodiment of course elements 1 and 2 or 41 and 42 must both be magnets,unless wall 43 is a magnet of opposite polarity instead of element 2 or42.

it will be understood that the devices shown are designed so that thehigher pressure of the two pressures being sensed is communicated to thepiston 1 or 41 via bore 12. When the magnetic elements are repelling,the higher pressure is communicated to the piston 1 or 41 via port 39. i

The terms react with and reaction with used in the claims refer to theforces of magnetic attraction or repulsion between the first and secondmagnetic means. The first magnetic means is spaced from and arranged .toattract or repel and thereby react with the second magnetic means, solong as the two means are within the mutually reactive magnetic fieldsof force. The term magnetic means encompasses both magnets andmagnetically attracted materials, and it will be understood that akeeper can be interposed therebetween, and that the keeper can be ofmagnetic or non magnetic material.

I claim: l. A magnetic pressure indicator which cannot be reset afteractuation without inverting the device, thus forestalling cancellationof the signal, comprising, in combination a first magnetic means spacedfrom and arranged to react with a second magnetic means so long as thetwo means are within their mutually reac tive magnetic fields of force;first bias means to retain the first magnetic means in a first positionwith respect to the second magnetic means, the second magnetic meansbeing normally retained in a second position; and second bias means topropel the second magnetic means from the second position to anindicating position whenever the spacing distance between the twomagnetic means is changed, the first magnetic means being movable inresponse to changes in pressure, and normally biased in the firstposition with respect to the second magnetic means by a predeterminedforce, and the second magnetic means being normally biased by the biasmeans to an indicating position by a force capable of overcoming theforce of magnetic reaction with the first means whenever the first andsecond magnetic means are spaced by a predetermined distance; a loosefree-rolling nonmagnetic ball normally nesting in a recess opening intothe path of the second magnetic means and retained in the recess by thesecond magnetic means while the means is magnetically reacted with thefirst magentic means, but whenever the second magnetic means moves adistance away from the first magnetic means sufficient to expose therecess, the ball rolls into the path of the second magnetic means andallows return of the second magnetic means only to a position where thespring-biasing force is still capable of overcoming the force ofmagnetic reaction between the second magnetic means and the firstmagnetic means, the recess being so arranged with respect to the secondmagnetic means that the ball cannot be returned to the recessby thesecond magnetic means in any normal orientation of the indicating deviceencountered under service, and the ball having a diameter greater thanthe distance within which the second magnetic means is reacted bymagnetic force to the first magnetic means.

2. A magnetic pressure indicator according to claim 1, in which thesecond magnetic means is in the form of a piston, moving in a bore intowhich the recess opens.

3. A magnetic pressure indicator according to claim 1, in which thesecond magnetic means is a piston of high surface area.

4. A magnetic pressure indicator according to claim 1, in which at leastone of the first and second bias means is a spring.

5. A magnetic pressure indicator according to claim 1, in which at leastone of the first and second bias means is a magnet.

6. A magnetic pressure indicator according to claim 1, in which therecess is in the second magnetic means.

7. A magnetic pressure indicator according to claim 1, having a wallbetween the first and second magnetic means and the recess disposed inthe wall opposite the second magnetic means.

8. A magnetic pressure indicator according to claim 1, in which thesecond magnetic means is arranged so as to becomevisible afteractuation. I

9. A magnetic pressure indicator according to clai 1, having the secondmagnetic means arranged so as to actuate a switch.

10. A magnetic pressure indicator according to claim 1, having abimetallic means arranged to prevent movement of at least one of thefirst and second magnetic means at temperatures below a predeterminedminimum.

11. A magnetic pressure indicator according to claim 1, in which boththe first and second magnetic means are magnets.

12. A magnetic pressure indicator according to claim 1, in which one ofthe first and second magnetic means is a magnet, and one is ofmagnetizable material.

13. A magnetic pressure indicator according to claim 1, in which thefirst and second magnetic means are separated by a wall, both of themagnetic means are of magnetizable material, and the wall is a magnet.

14. A magnetic pressure indicator according to claim 1, in which one ofthe first and second magnetic means is a magnet and the wall is amagnet.

15. A magnetic pressure indicator according to claim 1, in which thefirst and second magnetic means are mutually attracting.

16. A magnetic pressure indicator according to claim 1, in which thefirst and second magnetic means are mutually repelling.

17. A magnetic pressure indicator according to claim 1, in which thefirst magnetic means is spaced from and arranged to attract the secondmagnetic means, and the first bias means is arranged to retain the firstmagnetic means in a position toward the second magnetic means.

18. A magnetic pressure indicator according to claim 1, in which thefirst magnetic means is spaced from and arranged to repel the secondmagnetic means, and the first bias means is arranged to retain the firstmagnetic means in a first position away from the second magnetic means.

19. A magnetic pressure indicator according to claim 1, in which thefirst and second magnetic means are each spaced from and arranged toattract a keeper, and the first bias means is adapted to retain thefirst magnetic means in a first position towards the keeper, while thesecond bias means is adapted to propel the second magnetic means awayfrom the keeper.

20. A magnetic pressure indicator according to claim 1, in which thefirst magnetic means is spaced from and arranged to repel a keeper, andthe first bias means is arranged to retain the first magnetic means in afirst position away from the keeper, while the second magnetic means isarranged to attract the keeper, and the second bias means is adapted topropel the second magnetic means away from the keeper.

1. A magnetic pressure indicator which cannot be reset after actuationwithout inverting the device, thus foresTalling cancellation of thesignal, comprising, in combination a first magnetic means spaced fromand arranged to react with a second magnetic means so long as the twomeans are within their mutually reactive magnetic fields of force; firstbias means to retain the first magnetic means in a first position withrespect to the second magnetic means, the second magnetic means beingnormally retained in a second position; and second bias means to propelthe second magnetic means from the second position to an indicatingposition whenever the spacing distance between the two magnetic means ischanged, the first magnetic means being movable in response to changesin pressure, and normally biased in the first position with respect tothe second magnetic means by a predetermined force, and the secondmagnetic means being normally biased by the bias means to an indicatingposition by a force capable of overcoming the force of magnetic reactionwith the first means whenever the first and second magnetic means arespaced by a predetermined distance; a loose free-rolling nonmagneticball normally nesting in a recess opening into the path of the secondmagnetic means and retained in the recess by the second magnetic meanswhile the means is magnetically reacted with the first magentic means,but whenever the second magnetic means moves a distance away from thefirst magnetic means sufficient to expose the recess, the ball rollsinto the path of the second magnetic means and allows return of thesecond magnetic means only to a position where the spring-biasing forceis still capable of overcoming the force of magnetic reaction betweenthe second magnetic means and the first magnetic means, the recess beingso arranged with respect to the second magnetic means that the ballcannot be returned to the recess by the second magnetic means in anynormal orientation of the indicating device encountered under service,and the ball having a diameter greater than the distance within whichthe second magnetic means is reacted by magnetic force to the firstmagnetic means.
 2. A magnetic pressure indicator according to claim 1,in which the second magnetic means is in the form of a piston, moving ina bore into which the recess opens.
 3. A magnetic pressure indicatoraccording to claim 1, in which the second magnetic means is a piston ofhigh surface area.
 4. A magnetic pressure indicator according to claim1, in which at least one of the first and second bias means is a spring.5. A magnetic pressure indicator according to claim 1, in which at leastone of the first and second bias means is a magnet.
 6. A magneticpressure indicator according to claim 1, in which the recess is in thesecond magnetic means.
 7. A magnetic pressure indicator according toclaim 1, having a wall between the first and second magnetic means andthe recess disposed in the wall opposite the second magnetic means.
 8. Amagnetic pressure indicator according to claim 1, in which the secondmagnetic means is arranged so as to become visible after actuation.
 9. Amagnetic pressure indicator according to claim 1, having the secondmagnetic means arranged so as to actuate a switch.
 10. A magneticpressure indicator according to claim 1, having a bimetallic meansarranged to prevent movement of at least one of the first and secondmagnetic means at temperatures below a predetermined minimum.
 11. Amagnetic pressure indicator according to claim 1, in which both thefirst and second magnetic means are magnets.
 12. A magnetic pressureindicator according to claim 1, in which one of the first and secondmagnetic means is a magnet, and one is of magnetizable material.
 13. Amagnetic pressure indicator according to claim 1, in which the first andsecond magnetic means are separated by a wall, both of the magneticmeans are of magnetizable material, and the wall is a magnet.
 14. Amagnetic pressure indicator according to claim 1, in which one of thefirst and second magnetic means is a magnet and the wAll is a magnet.15. A magnetic pressure indicator according to claim 1, in which thefirst and second magnetic means are mutually attracting.
 16. A magneticpressure indicator according to claim 1, in which the first and secondmagnetic means are mutually repelling.
 17. A magnetic pressure indicatoraccording to claim 1, in which the first magnetic means is spaced fromand arranged to attract the second magnetic means, and the first biasmeans is arranged to retain the first magnetic means in a positiontowards the second magnetic means.
 18. A magnetic pressure indicatoraccording to claim 1, in which the first magnetic means is spaced fromand arranged to repel the second magnetic means, and the first biasmeans is arranged to retain the first magnetic means in a first positionaway from the second magnetic means.
 19. A magnetic pressure indicatoraccording to claim 1, in which the first and second magnetic means areeach spaced from and arranged to attract a keeper, and the first biasmeans is adapted to retain the first magnetic means in a first positiontowards the keeper, while the second bias means is adapted to propel thesecond magnetic means away from the keeper.
 20. A magnetic pressureindicator according to claim 1, in which the first magnetic means isspaced from and arranged to repel a keeper, and the first bias means isarranged to retain the first magnetic means in a first position awayfrom the keeper, while the second magnetic means is arranged to attractthe keeper, and the second bias means is adapted to propel the secondmagnetic means away from the keeper.